Title: Overcoming the Photovoltage Plateau in Large Bandgap Perovskite Photovoltaics

Abstract

Development of large bandgap (1.80-1.85 eV Eg) perovskite is crucial for perovskite-perovskite tandem solar cells. However, the performance of 1.80-1.85 eV Eg perovskite solar cells (PVKSCs) are significantly lagging their counterparts in the 1.60-1.75 eV Eg range. This is because the photovoltage (Voc) does not proportionally increase with Eg due to lower optoelectronic quality of conventional (MA,FA,Cs)Pb(I,Br)3 and results in a photovoltage plateau (Voc limited to 80% of the theoretical limit for ~1.8 eV Eg). Here, we incorporate phenyl- ethylammonium (PEA) in a mixed-halide perovskite composition to solve the inherent material-level challenges in 1.80-1.85 eV Eg perovskites. The amount of PEA incorporation governs the topography and optoelectronic properties of resultant films. Detailed structural and spectroscopic characterization reveal the characteristic trends in crystalline size, orientation, and charge carrier recombination dynamics and rationalize the origin of improved material quality with higher luminescence. With careful interface optimization, the improved material characteristics were translated to devices and Voc values of 1.30-1.35 V were achieved, which correspond to 85-87% of the theoretical limit. Using an optimal amount of PEA incorporation to balance the increase in Voc and the decrease in charge collection, a highest power conversion efficiency of 12.2% was realized. Our results clearly overcomemore » the photovoltage plateau in the 1.80-1.85 eV Eg range and represent the highest Voc achieved for mixed-halide PVKSCs. This study provides widely translatable insights, an important breakthrough, and a promising platform for next- generation perovskite tandems.« less

Figures / Tables:

Figure 1: Comparison of the figure-of-merit (open-circuit voltage, Voc) for mixed-halide hybrid PVKSCs reported in literature in perspective to this work. (a) Voc and (b) corresponding Voc/Voc,SQ as a function of bandgap (Eg) for works in the 1.6−1.9 eV Eg range. Red stars correspond to values obtained in this work.more » Citations for points 1−22 and an associated discussion are provided in Supporting Information.« less

The development of stable high-bandgap hybrid perovskites (HPs) with high optoelectronic quality may enable tandem solar cells with power conversion efficiencies approaching 30%. The halide composition of HPs has been observed to effect bandgap, carrier lifetime, and material stability. Here we report optoelectronic quality and stability under illumination of thousands of compositions ranging from the pure iodide (CH3NH3PbI3) to the diiodomonobromide (CH3NH3PbI2Br). Hyperspectral maps of steady-state absolute intensity photoluminescence (AIPL) are used to determine the quasi-Fermi level splitting (QFLS) at each point after synthesis. The QFLS upon first illumination increases with bandgap and reaches a maximum of 1.27 eV undermore » 1 sun illumination intensity for a bandgap of 1.75 eV. However, the optoelectronic quality (χ), defined as the ratio of the QFLS to the maximum theoretical QFLS for bandgap, decreases with bandgap from around 88% for 1.60 eV bandgap down to 82% for 1.84 eV bandgap. Further, we show that a reversible light induced defect forms that reduces the optoelectronic quality, particularly for high-bandgap materials. Composition analysis shows that the halide to lead ratio, (I + Br)/Pb, decreases from 3 for the pure iodide to 2.5 for the diiodomonobromide, suggesting a role of halide vacancies or halide substitution defects in the light-induced instability for this synthesis route. Even with the light-induced defect, a stable QFLS of about 1.17 eV is possible. Comparing our QFLS to Voc values from HP devices reported in the literature indicates that higher open circuit voltages are possible but may require optimization of band alignment. Further, the spectral shape of the PL emission is found to be more commensurate with Franz–Keldysh broadening from local electric fields or from a screened Thomas–Fermi density of states (as opposed to a joint density of states due to Urbach disorder).« less

Multijunction solar cells based on epitaxially grown III-V materials hold the record for solar energy power conversion efficiency (PCE). However, due to the high cost of fabricating these devices, they are typically only used for concentrator cells and space applications. The overarching goal of this project was to develop low-cost printable hybrid perovskite (HP) materials appropriate and optimized for tandem solar cells with high power conversion efficiency under “1 Sun” illumination. Key results and findings over the course of the project we: Developed higher-performance high-bandgap (1.75 eV) perovskite materials and devices. In particular, we explored tens-of-thousands of compositions for highmore » bandgap perovskites, achieving quasi-Fermi level splitting of 1.35 eV for a 1.75 eV bandgap material. We achieved World-record open circuit voltages from single junction p-i-n devices, 1.24 V from 1.75 eV bandgap material, which is what is preferable for tandems with a PCE of 14.3% using a guanidinium/formanadinium/ cesium alloyed lead iodobromide. We also developed a series of World-record efficiency devices at higher band-gaps based on 2D/3D perovskites using PEA. Developed higher-performance low-bandgap (1.35 eV) perovskite materials and devices. In particular, we developed a 1.35 eV bandgap perovskite of composition MAPb0.5Sn0.5(I0.8Br0.2)3 and showed its superiority to MAPb0.75Sn0.25I3. High efficiency solar cells were fabricated using PEDOT:PSS and doped-ICBA as HTL and ETL, respectively. Short circuit currents of 25.7 mA/cm2 and PCEs of 17.1% were obtained. Developed mechanically stacked 4-terminal CIGS-Perovskite tandems with PCE of 18.8% and monolithic 2-terminal CIGS-Perovskite tandems with PCE of 8.5%. The low efficiency of the monolithic device is a result of the high surface roughness of the solution processed CIGS bottom cells. This is not an intrinsic problem for CIGS-perovskite tandems, but does mean that smooth evaporated or sputtered CIGS films likely need to be used, unless a polishing step is employed. Developed monolithic 2-terminal Perovskite-Perovskite tandems with a stabilized PCE of 18.5%. This was the World-record perovskite-perovskite monolithic tandems for over a year in 2017-2018. Revealed that light is not an essential component of the so-called “light-induced” phase segregation. By using charge injection in the dark and electroluminescence, we showed that the presence of electrons in the conduction band and hole in the valence band is sufficient to drive the nearly ubiquitously observed phase segregation in high bandgap perovskites. Developed a new method to simultaneously measure absolute intensity photoluminescence and photoconductivity and use them to obtain simultaneous in-situ measurement of quasi-Fermi level splitting and diffusion length. This is important since it provides a proxy for device Voc and device Jsc. In addition, 67 papers were published with support from this award that detail many more advances in the field, including numerous publications in high impact journals such as Nature Photonics, Advanced Materials, ACS Energy Letters, and Energy and Environmental Science.« less

Tin-lead perovskites have low band gaps (1.2 eV - 1.4 eV) that offer complementary absorption spectra to mixed halide perovskites with band gaps in the range of 1.6 eV - 1.85 eV. In combination, tin-lead and mixed halide perovskites can be used to make efficient all-perovskite tandem solar cells. While these can achieve high efficiency, they are hampered by the unproven longterm stability of tin-containing perovskites. We make the first demonstration of tin-lead perovskite solar cells that pass benchmark 1000-hour tests of stability under stressors of heat, light, and atmospheric exposure. We identify that mixed tin-lead perovskites oxidise by amore » different chemical pathway, one that is much less favourable, than the oxidation pathway followed by pure tin perovskites, making them significantly more stable. Fortuitously, mixed tin-lead perovskites also have band gaps that are close to ideal for the rear cell of an all-perovskite monolithic two-junction tandem. Beyond selecting the composition of the perovskite to be one that has inherently suppressed tendency to oxidise, we also fabricate a novel heterojunction-based architecture that eliminates the commonly used acidic PEDOT:PSS layer, which leads to enhanced stability. By packaging the resulting solar cell in a glass-on-glass package with a pliable low-elastic-modulus polyolefin encapsulant and a butyl rubber edge seal, we also demonstrate that the low band gap perovskite solar cell that passes the IEC damp heat test for thin film solar cells - a milestone that demonstrates the removal of a significant impediment to the commercialization of high efficiency perovskite tandem solar cells.« less

We report the development of GaInP/GaAs monolithic tandem solar cells grown by hydride vapor phase epitaxy (HVPE). HVPE is a route to reduced III-V growth costs because the technique takes advantage of low cost inputs (elemental metals and HCl) and high source utilization, and exhibits high growth rates (up to 5 Au/min). The tandem device consists of three main components: a 1.90 eV Ga0.5In0.5P top cell, a p-Ga0.5In0.5P/n-GaAs tunnel junction, and a 1.41 eV rear heterojunction GaAs cell. The open circuit voltage (VOC) of the tandem is 2.41 V, indicating high material quality in both subcells, and voltage addition throughmore » the tunnel junction. Electroluminescence measurements indicate that the individual VOC's of the top and bottom cells are 1.40 and 1.01 V, respectively, at short circuit. This yields a band gap (EG) voltage offset, WOC = EG/q-VOC = 0.50 and 0.40 V, respectively, where q is the elementary charge. The WOC of the top cell is higher in part because the structure contains an unpassivated front surface, and because the back surface field (BSF) consists of p+ GaInP rather than a higher- EG heterobarrier. The top cell limits the current of this series-connected device for these reasons, with a short-circuit current density, JSC, of 10.1+/- 0.2 mA/cm2. The overall efficiency is 20.3 +/- 0.4% (uncertified). We measured the device under concentration to look for signs of tunnel junction breakdown, which we did not observe up to ~1000x, which was the highest concentration tested. Importantly, our dynamic-HVPE reactor enables us to deposit this device, which requires multiple abrupt changes in composition and doping, in a short growth time. We discuss next steps to improve the current result, with a clear pathway towards 30% efficiency. The potential for terrestrial applications of this lower cost III-V technology is also discussed.« less

The first monolithic perovskite/silicon tandem was made with a diffused silicon p-n junction, a tunnel junction made of n ++ hydrogenated amorphous silicon, a titania electron transport layer, a methylammonium lead iodide absorber, and a Spiro-OMeTAD hole transport layer (HTL). The power conversion efficiency (PCE) was only 13.7% due to excessive parasitic absorption of light in the HTL, limiting the matched current density to 11.5 mA/cm 2. Werner et al. 15 raised the PCE to a record 21.2% by switching to a silicon heterojunction bottom cell and carefully tuning layer thicknesses to achieve lower optical loss and a higher currentmore » density of 15.9 mA/cm 2. It is clear from these reports that minimizing parasitic absorption in the window layers is crucial to achieving higher current densities and efficiencies in monolithic tandems. To this end, the window layers through which light first passes before entering the perovskite and silicon absorber materials must be highly transparent. The front electrode must also be conductive to carry current laterally across the top of the device. Indium tin oxide (ITO) is widely utilized as a transparent electrode in optoelectronic devices such as flat-panel displays, smart windows, organic light-emitting diodes, and solar cells due to its high conductivity and broadband transparency. ITO is typically deposited through magnetron sputtering; however, the high kinetic energy of sputtered particles can damage underlying layers. In perovskite solar cells, a sputter buffer layer is required to protect the perovskite and organic carrier extraction layers from damage during sputter deposition. The ideal buffer layer should also be energetically well aligned so as to act as a carrier-selective contact, have a wide bandgap to enable high optical transmission, and have no reaction with the halides in the perovskite. Additionally, this buffer layer should act as a diffusion barrier layer to prevent both organic cation evolution and moisture penetration to overcome the often-reported thermal and environmental instability of metal halide perovskites. Previous perovskite-containing tandems utilized molybdenum oxide (MoO x) as a sputter buffer layer, but this has raised concerns over long-term stability, as the iodide in the perovskite can chemically react with MoO x. Mixed-cation perovskite solar cells have consistently outperformed their single-cation counterparts. The first perovskite device to exceed 20% PCE was fabricated with a mixture of methylammonium (MA) and formamidinium (FA). Recent reports have shown promising results with the introduction of cesium mixtures, enabling high efficiencies with improved photo-, moisture, and thermal stability. The increased moisture and thermal stability are especially important as they broaden the parameter space for processing on top of the perovskite, enabling the deposition of metal oxide contacts through atomic layer deposition (ALD) or chemical vapor deposition (CVD) that may require elevated temperatures or water as a counter reagent. Both titanium dioxide (TiO 2) and tin oxide (SnO 2) have consistently proven to be effective electron-selective contacts for perovskite solar cells and both can be deposited via ALD at temperatures below 150 °C. We introduced a bilayer of SnO 2 and zinc tin oxide (ZTO) that can be deposited by either low-temperature ALD or pulsed-CVD as a window layer with minimal parasitic absorption, efficient electron extraction, and sufficient buffer properties to prevent the organic and perovskite layers from damage during the subsequent sputter deposition of a transparent ITO electrode. We explored pulsed-CVD as a modified ALD process with a continual, rather than purely step-wise, growth component in order to considerably reduce the process time of the SnO 2 deposition process and minimize potential perovskite degradation. These layers, when used in an excellent mixed-cation perovskite solar cell atop a silicon solar cell tuned to the infrared spectrum, enable highly efficient perovskite-silicon tandem solar cells with enhanced thermal and environmental stability.« less